Most physiological functions, including breathing and heartbeat as well as other muscular actions, involve internal bodily motions that are uniquely related thereto. These motions include such actions as contraction of muscles, expansion of lungs, opening and closure of cardiac and other valves, and expansion and contraction of arteries in response to voluntary or involuntary physiological actions. Some of the larger scale of these internal motions, such as heart and respiratory functions, have heretofore been monitored by complicated equipment employing, inter alia, microwave or radio frequency Doppler radar systems. Systems of this kind are found in U.S. Pat. Nos. 4,638,808, 4,991,585, and 4,967,751.
In U.S. Pat. No. 4,638,808, for example, there is disclosed a microwave radar system capable of detecting a patient's heart rate. The system includes an antenna that is positionable adjacent a patient's thorax. The antenna both transmits microwave radiation into a patient's chest and receives therefrom Doppler shifted microwave signals representative of a combination of a patient's respiratory and cardiac activity. That component of the received signal which corresponds to respiration is extracted via a signal component separating circuit, whereby the ultimately displayed signal is reflective of the patient's heart rate.
U.S. Pat. No. 4,991,585 provides a triple antenna microwave radar system for measuring a patient's respiration and/or heartbeat. The triple antenna is placed adjacent a patient's thorax to transmit into and receive microwave radiation therefrom. One of the antennas is a microwave transmitter, a second is a microwave transducer and a third is a microwave receiver. Through elaborate circuitry the system processes microwave radiation reflected from objects both inside an outside of the patient's body, extracts spurious signals, and outputs a representative respiratory and/or heartbeat signal to an alarm-control means.
A related system is described in U.S. Pat. No. 4,967,751. According to this patent, however, a pair of cooperating microwave transducers are situated at opposite side s, i.e. front and back, of a patient's thorax to derive the desired heartbeat and/or respiratory signal. Although effective, the requirement of two transducer modules complicates design, manufacture and utilization of the system.
Some other physiological motions result in small, but significant, rearrangements of certain localized internal structure of the body. For example, periodic expansion of arteries responsive to blood flow is a motion indicative of a patient's pulse rate, a commonly monitored and important physiological function parameter. As with cardiac and respiratory activity, Doppler radar and other monitoring systems for pulse rate are also known.
In this connection, U.S. Pat. No. 4,753,243 discloses a pulse rate monitor comprising first and second units coupled by a cable. The first unit comprises, inter alia, a DC microwave oscillator and an antenna contained in a grounded case. In operation, the first unit preferably contacts the patient and is disposed at a desired location, e.g., with the antenna proximate an artery. The first unit is then activated to emit an oscillating DC signal. As the artery expands and contracts with each heart beat, the loading on the antenna and, therefore, the oscillator, changes. In turn, this causes the oscillating signal to be "pulled" into a pulsating signal in accordance with the pulse rate to be conveyed by the cable to the second unit. The second unit is typically connected to output means where the pulse rate signal can be seen and/or heard. While such a system enhances patient comfort through miniaturization of the first unit, the patient nonetheless remains encumbered by the potentially lengthy connector cable that is apt to become caught on bed rails, lavatory equipment, food trays, intravenous (IV) poles and other objects frequently encountered by a hospitalized patient. Consequently, either the first unit may become dislodged from the patient or the cable may disconnect from the second unit, thereby interrupting signal transmission.
An advantage exists, therefore, for a physiological sensor using directly radiated, motion induced oscillator frequency changes and a wireless telemetry system incorporating such a sensor.